1. Field of the Invention
The present invention relates to an optical transmission line in which a plurality of optical amplifiers are connected in series with optical fibers at a predetermined distance to amplify a wavelength multiplexed optical signal transmitted over the optical fibers in conformity with a wavelength multiplexing communication system. More particularly, the present invention relates to an optical transmission line constructed to compensate for wavelength-dependent gain characteristics of the plurality of optical amplifiers using an erbium-doped optical fiber as gain medium and to optimally equalize an overall gain of the optical amplifiers.
2. Description of the Prior Art
As an optical amplifier to amplify a wavelength multiplexed optical signal transmitted over an optical fiber in conformity with the wavelength multiplexing communication system, an erbium-doped optical fiber amplifier (abbreviated as an "EDF amplifier" hereinafter) has been used. Although the EDF amplifier may achieve such an advantage that it can amplify optical signals in multiple channels simultaneously without mutual interference, it is difficult for the ordinary EDF amplifier to achieve flat gain of the optical signal throughout a wide wavelength range. In other words, if the EDF amplifier is used in the wavelength multiplexing communication system, a wide and flat gain bandwidth is required of the EDF amplifier. However, since it is difficult for the EDF amplifier to attain completely flat gain over the wide wavelength range, significant unevenness of gain is caused if such EDF amplifiers are connected to optical transmission lines consisting of optical fibers in a multistage fashion. In order to compensate for such unevenness in the gain characteristic of the EDF amplifiers, various approaches have been proposed in the prior art.
FIG.1 is a view showing a configuration of a Mach-Zehnder variable-wavelength filter (referred to as an "MZ type variable-wavelength filter" hereinafter) to equalize gain of the EDF amplifiers by compensating for unevenness of gain of the EDF amplifiers. The MZ type variable-wavelength filter is so constructed that a heater 97 is attached to one of two waveguides 91, 92 having different lengths, two waveguides 91, 92 are sandwiched by two tunable couplers 93, 94, and heaters 95, 96 constituting a phase shifter are provided to one of two waveguides 91, 92. (See Kyo Inoue, Toshimi Kominato, and Hiromu Toba, "Tunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifier", IEEE Photonics Technology Letters, Vol. 3, No. 8, pp. 718-720, August 1991.)
The MZ type variable-wavelength filter constructed as above has a sinewave function type transmission characteristic having a period which is determined according to difference in lengths of two waveguides 91, 92. A center frequency of the transmission characteristic can be shifted by the phase shifter constructed by the heater 97. Still further, an extinction ratio of the transmission characteristic can be varied by the phase shifter consisting of the heaters 95, 96. Therefore, as shown in FIGS. 2A to 2C, the transmission characteristic in a certain wavelength range can be varied optimally. Therefore, if the transmission characteristic of the MZ type variable-wavelength filter is designed to have equal gain of the wavelength multiplexed optical signal in respective wavelengths and then the MZ type variable-wavelength filter is connected to the output side of the optical amplifier, gain of the optical amplifier can be equalized.
In the meanwhile, an erbium-doped optical fiber in which aluminum (Al) as well as erbium are co-doped is used in the ordinary EDF amplifier. In this case, if the wavelength multiplexed optical signal is amplified by the EDF amplifier, the optical signal has an output optical spectrum characteristic, as shown in FIG. 3A. Optical power of the wavelength multiplexed optical signal in respective wavelengths increases upward to the right in an appropriate wavelength range. The characteristic in FIG. 3A shows a case where four wavelength multiplexed optical signals are transmitted at their full input power of -15 dBm.
In contrast, if the wavelength multiplexed optical signal is amplified by the EDF amplifier composed of an erbium-doped optical fiber in which phosphorus (P) and aluminum (Al) as well as erbium are co-doped, optical power of the wavelength multiplexed optical signal in respective wavelengths decreases downward to the right in an appropriate wavelength range.
Accordingly, as shown in FIG. 4, if a hybrid EDF amplifier is so constructed that the erbium-doped optical fiber 97 in which aluminum (Al) as well as erbium is co-doped and the erbium-doped optical fiber 98 in which phosphorus (P) and aluminum (Al) as well as erbium are co-doped are connected serially and then an output laser beam emitted from a pump laser diode module 100 is introduced into both optical fibers 97 and 98 via a WDM coupler 99 to excite them, the right-upward characteristic of the optical fiber 97 and the right-downward characteristic of the optical fiber 98 can be canceled mutually. As a result, as shown in FIG. 3C, the output optical spectrum characteristic of the hybrid EDF amplifier becomes flat, thus equalizing gain throughout the wavelengths. (See T. Kashiwada, M. Shigematu, M. Onishi and M. Nishimura, "Gain-Flattened Optical-Fiber Amplifiers with a Hybrid Er-doped-Fiber Configuration for WDM Transmission", OFC'95, TuPl, 1995.)
As mentioned above, in the conventional approach in which gain of the optical amplifier can be equalized by connecting the MZ type variable-wavelength filter to the output side of the optical amplifier, there are problems that the optical filter is complicated in structure so that the device is increased in size and the cost of production is also increased.
In addition, in the conventional approach in which the hybrid EDF amplifier is constructed by connecting the erbium-doped optical fiber in which aluminum (Al) as well as erbium is co-doped and the erbium-doped optical fiber in which phosphorus (P) and aluminum (Al) as well as erbium are co-doped are connected in series with each other, there are problems that, when the hybrid EDF amplifiers are connected in a multistage fashion, ASE (amplified spontaneous emission) in a 1.53 .mu.m wavelength range is increased so that gain in a signal wavelength range of 1.55 .mu.m cannot be achieved.
FIGS. 5A to 5C are characteristic diagrams in which ASE of a light signal supplied from the EDF amplifier using the erbium-doped optical fiber in which phosphorus (P) as well as erbium is co-doped to increase ASE in the 1.53 .mu.m wavelength range is plotted as a function of wavelength. FIG. 5A shows ASE characteristic obtained in the event that three EDF amplifiers using the erbium-doped optical fiber in which phosphorus (P) together with erbium is co-doped are connected and the optical signal is transmitted by a transmission distance of 120 km. FIG. 5B shows ASE characteristic obtained in the event that twenty-seven EDF amplifiers are connected and the optical signal is transmitted by a transmission distance of 1000 km. FIG. 5C shows ASE characteristic obtained in the event that a hundred and fifty EDF amplifiers are connected and the optical signal is transmitted by a transmission distance of 6000 km.
From the ASE characteristics shown in FIGS. 5A to 5C, it can be appreciated that, if the more EDF amplifiers using the erbium-doped optical fiber in which phosphorus (P) as well as erbium is co-doped that are connected in a multistage fashion, the more ASE in the 1.53 .mu.m wavelength range has been increased, whereby gain of the signal in the wavelength range of 1.55 .mu.m has not been able to be obtained.